Iron oxyhydroxides, such as goethite and ferrihydrite, are highly abundant and ubiquitous minerals in geochemical environments. Because of their small particle sizes, their surface reactivity is high towards adsorption of anions and cations of environmental relevance. For this reason these minerals are extensively studied in environmental geochemistry, and also are very important for environmental and industrial applications. In the present work, we report the synthesis and characterization of goethite and ferrihydrite of controlled particle sizes. It has been shown that surface reactivity of these minerals is highly dependent on crystal sizes, even after normalizing by specific surface area. In order to investigate the reasons for this changing reactivity it is necessary to work with reproducible particle sizes of these minerals. We investigated here the experimental conditions to synthesize goethite samples of four different specific surface areas: ca. 40, 60, 80 and 100 m 2 g -1 , through the controlled speed of hydroxide addition during hydrolysis of acid Fe(III) solutions. In the case of 2-line ferrihydrite, samples with two different particle sizes were prepared by changing the aging time under the pH conditions of synthesis (pH = 7.5). The synthesized minerals were identified and characterized by: X-ray diffraction, N 2 adsorption BET specific surface area, transmission electron microscopy, attenuated total reflectance Fourier transform infrared spectroscopy, and maximum Cr(VI) adsorption.Keywords: synthesis, iron oxides, specific surface area, goethite, ferrihydrite, particle size. Resumen

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Lead ion in drinking water is one of the most dangerous metals. It affects several systems, such as the nervous, gastrointestinal, reproductive, renal, and cardiovascular systems. Adsorption process is used as a technology that can solve this problem through suitable composites. The adsorption of lead (Pb(II)) on graphene oxide (GO) and on two goethite (α-FeOOH)/reduced graphene oxide (rGO) composites (composite 1: 0.10 g GO: 22.22 g α-FeOOH and composite 2: 0.10 g GO: 5.56 g α-FeOOH), in aqueous medium, was studied. The GO was synthesized from a commercial pencil lead. Composites 1 and 2 were prepared from GO and ferrous sulfate. The GO and both composites were characterized by using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), Raman spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and dynamic light scattering (DLS). The adsorption capacity of Pb(II) on the GO and both composites was evaluated through adsorption isotherms. Composite 1 presented a significant agglomeration of α-FeOOH nanorods on the reduced graphene oxide layers. Meanwhile, composite 2 exhibited a more uniform distribution of nanorods. The adsorption of Pb(II) on the three adsorbents fits the Langmuir isotherm, with an adsorption capacity of 277.78 mg/g for composite 2200 mg/g for GO and 138.89 mg/g for composite 1. Composite 2 emerged as a highly efficient alternative to purify water contaminated with Pb(II).

Most dyes used in the textile industry are chemically stable and poorly biodegradable, therefore, they are persistent in the environment and difficult to degrade by conventional methods. An alternative treatment for this kind of substance is heterogeneous photocatalysis using TiO2, so, in this work, it is proposed to degrade Direct Blue 1 (DB1) using microparticulate TiO2 irradiated with e-beam at three different doses: 5, 10 and 20 kGy (J/kg). The DB1 degradation was implemented in a batch reactor (DB1 initial concentration = 50 mg L−1, pH 2.5, TiO2 concentration = 200 mg L−1). We have demonstrated that the photocatalytic power of TiO2, when irradiated with e-beam (5, 10, 20 kGy), varies slightly, with minor effects on photodegradation performance. However, the dose of 10 kGy showed a slightly better result, according to the DB1 photodegradation rate constant. Adsorption process was not affected by irradiation; its isotherm was fitted to Freundlich’s mathematical model. The DB1 photodegradation rate constants, after one hour of treatment, were: 0.0661 and 0.0742 min−1 for irradiated (10 kGy) and nonirradiated TiO2, respectively. The degradation rate constant has an increase of 12.3% for irradiated TiO2. Finally, there was no evidence of mineralization in the degradation process after 60 min of treatment. According to the results, the irradiation of microparticulate TiO2 with e-beam (10 kGy) slightly improves the photodegradation rate constant of DB1.

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